1. Field of the Invention
The present invention relates to a resilient back-up mechanism for a vehicle seat structure. More particularly, the present invention pertains to an adjustable resilient back-up mechanism for supporting the waist portion of a passenger sitting on the seat.
2. Description of the Related Art
There are several types of structures for making the resilient back-up mechanisms for vehicle seats adjustable. The known types include a direct drive system as shown by the Japanese Patent Publication No. 56-10051, a cam drive type as disclosed by the Japanese Patent Publication No. 55-49 and a screw drive type as disclosed by the Japanese utility model publication 51-20802. The direct drive is of a mechanism having a back-up member located at a portion corresponding to the waist portion of the passenger sitting on the seat. An actuating lever is provided for moving the back-up member to thereby adjust the back-up force. This mechanism is however disadvantageous in that a large actuating force is required when the adjustment is carried out while the passenger's weight is applied on the seat back. Further, the mechanism is disadvantageous in that a complicated ratchet mechanism must be provided for holding the parts of the mechanism in desired positions. The ratchet mechanism is mounted on the seat back frame. However, it is a troublesome work to install the ratchet mechanism during a seat assembling process.
The cam drive type has a cam which is actuated by an actuating lever to thereby adjust the position of the back-up member. This type is also disadvantageous in that a large actuating force is required when an adjustment is made while the weight of the passenger is applied on the seat back. Another disadvantage of this type is that a preassembled adjusting mechanism is mounted by means of bolts on brackets which are provided on the seat back frame, so that the mounting procedure is very troublesome.
The screw drive type includes a screw-nut mechanism for converting a rotation of a handle member to an axial movement. This mechanism is inconvenient to use because a lot of rotation is required to obtain a desired adjustment. If a coarse thread is adapted to increase the adjusting speed, an increased operating force will be required.